Holography is an image-recording process distinct from other image-recording processes; both the phase and amplitude of, for example, a coherent wavefront "modified" by interaction with a three-dimensional object (i.e., "interrupted"), is recorded onto a recording medium. The phase and amplitude information thus stored can be reconstructed (i.e., "played back") to provide a three-dimensional image of the original three-dimensional object.
In the production of holograms in general, an object to be imagewise recorded is irradiated with a first component split from a coherent radiation source (e.g., from a laser). Irradiation reflected from the object is directed toward an appropriately sensitized recording medium (e.g., a photographic plate or a photopolymeric medium). A beam of such object-interrupted coherent radiation is commonly termed an object beam. At the same time, a second component split from the coherent radiation source is directed to the recording medium, bypassing the object A beam of such coherent radiation is commonly termed a reference beam. The interference pattern resultant of the interaction of the reference beam and the object beam contemporaneously impinging on the recording medium is latently recorded in the recording medium. When the exposed recording medium is processed (e.g., for development of the latent recordation) and subsequently appropriately irradiated and observed at an appropriate angle (i.e., generally an angle correspondent with the incident angle of the reference beam), the irradiation interacts with the interference pattern (cf., the hologram) to reconstruct the wavefront that originally reached the recording medium as reflected from the object. A holographically reconstructed image is produced.
With evolution of the underlying technology, the practice of holography has proliferated into several and diverse applications. Among such applications, the utilization of reflective display holograms in pendants, rings, belt buckles, action figures, novelties, and the like, has generated considerable consumer interest. Commercial interest in sunglasses (and other eyewear) embodying display holograms has also emerged. But, development of sunglasses carrying an imprinted display hologram has been frustrated by the need to still maintain adequate visibility though the sunglasses.
In attempts to resolve such problems, recent proposals have suggested use of embossed reflective display holograms.
An embossed reflective display hologram is typically formed from an off-axis master hologram in a multi-step process. The first step usually involves making the master off-axis hologram where the real object is positioned some distance from the surface of the recording medium and the reference beam is a collimated or parallel beam. The second step usually involves illuminating the master off-axis hologram with a collimated beam of light to project a real image of the object into space. A second hologram is then made by positioning a new recording medium at the position of the projected real image and by introducing a new reference beam at an angle. In making embossed holograms, the recording medium used in this second step is typically a holographic photoresist. A holographic photoresist is material which, when holographically exposed and developed, yields a surface profile whose depth is proportional to the intensity of the incident irradiation. The third step of making an embossed hologram usually involves coating the surface of the holographic photoresist exposed in the second step with a conducting metal, such as silver, then immersing the coated hologram in an electroplating bath to plate a layer, such as a layer of nickel, thereon. The fourth step involves using the nickel plate layer as a hard master to emboss the interference pattern into plastic that has been softened by heat, pressure, solvents, or some combination thereof in a continuous fashion. Finally, in the last step, after embossing, the plastic is typically coated with a highly reflecting metal, like aluminum, to enhance the reconstruction efficiency of the embossed hologram.
In the aforementioned proposals to utilize embossed reflective display holograms for sunglasses, it was found that by replacing the "highly reflecting metal" of conventional embossed holograms with a partially reflecting metal (or like composition), one could produce sunglasses having some holographic functionality while maintaining some degree of visibility therethrough. Accordingly, the specification of U.S. Pat. No. 4,315,665 (Haines) proposes a composite optical element which comprises a first layer or substrate bearing holographic information in the form of a surface pattern, a thin coating which conforms to the surface pattern and is either partially reflective or is of a transparent material of which the reflection index is different from the reflective index of said substrate and third layer which fills in the surface variations of said coating, the third layer being optically transparent, having a refractive index equal to that of the first layer. Likewise, U.S. Pat. No. 4,840,444 (Hewitt) proposes a composite holographic element comprising a substrate having on one surface a relief pattern providing a holographic image, a partially reflective layer facing said surface, and a thin optically absorbent layer adjacent the substrate.
In considering the use of embossed reflective display holograms for sunglasses, certain observations are made. First, it is believed that, in general, a holographic image produced in reflection by such an element will be undesirably dim. Moreover, if the thin reflective coating (or intermediate layer) is increased in thickness to enhance the brightness of such a hologram the desirable optical performance of the sunglasses in other respects may be degraded. While aforementioned U.S. Pat. No. 4,840,440 (Hewitt), attempts to resolve these problems, neither Hewitt nor Haines address a central drawback of such sunglasses: i.e., reliance on a partially reflecting layer (or the like) to accomplish desirable holographic functionality. Use of such layer narrows the product's configuration and appearance (cf., "mirror-like" sunglasses), as well as introduces complex electroplating processes (or the like) into its manufacture.
The present invention departs from the proposed utilization of embossed reflective display holograms, and instead, focuses on and draws upon the technology relating to volume phase reflection holograms. While the general concept of using volume phase reflection holograms as such is not new (see UK Patent Application GB 2 159 975 A, published Dec. 11, 1985), the known prior art methods of making such articles cannot be easily implemented for mass production and/or compromise product quality.
Briefly, in forming a volume phase reflection hologram, an object beam and a reference beam impinge upon an appropriate recording medium from opposite sides (see e.g., FIG. 2b), with planes of resulting interference fringes being formed substantially parallel to the surface of the recording medium. The planes are spaced apart within the recording medium at a distance which is generally equal to one-half the wavelength of the recording light divided by the index of refraction of the recording medium. Typical recording media used in the art are fine grained silver halide emulsions (for which the interference fringe planes comprise regions of high density of developed silver) or dichromated gelatin or photopolymer (for which the interference fringe planes comprise regions of slight differences in the index of refraction in comparison with lower exposed regions). When a volume phase reflection hologram is illuminated with white light, only light having the same wavelength as that of the light that was used in recording is reflected back to the viewer. While the present inventors do not wish to be limited to any theory in explanation of their invention, it is believed that this occurs because the interference fringe planes that are stacked a half wavelength apart will only coherently backscatter light of that wavelength, i.e., they allow constructing interference. All other wavelengths destructively interfere and are scattered out of the field of view because they do not match the spacing of the planes.
In an application of such technology, the present inventors have developed a methodology--well suited for mass production--that provides desirable utilization of volume phase reflection holograms in, for example, sunglasses, goggles, helmet visors, novelty eyewear, and the like. The resultant articles are durable and of good quality. Further, good reconstruction efficiencies are achieved without the requirement of a reflective metallic coating. Unburdened by electroplating processes (and like processes), the articles may be manufactured more economically and with greater variety.